(a) Field of the Invention
The present invention relates to polarization mode converter having a diffraction grating, for use in polarization mode conversion between TE mode light and TM mode light.
(b) Description of the Related Art
An optical waveguide-type polarization mode converter is known which is capable of being integrated with semiconductor lasers to form an optical semiconductor device. FIG. 1A is a schematic sectional view of an example of conventional TE/TM mode converters of this type. The TE/TM mode converter has a structure similar to that of a semiconductor laser of a combined resonator structure and is called as such.
The TE/TM converter has a multiple quantum well (MQW) resonator structure wherein the resonator is divided in the longitudinal direction thereof into two regions 51 and 52. The first region 51 has a compressive-strained MQW structure wherein the compressive strain is introduced in the quantum well active layer, whereas the second region 52 has a tensile-strained MQW structure wherein tensile strain is introduced in the quantum well active layer, both region 51 and region 52 having separate electrodes.
When a compressive strain or tensile strain is introduced into a quantum well active layer, the band structure of the quantum well is generally changed. The first valence band level of compressive-strained MQW resides in a HH band, which causes amplification or absorption for TE mode light. On the other hand, the first valence band level of tensile-strained MQW resides in a LH band, which causes amplification or absorption for TM mode light. Amplification or absorption can be controlled by the bias voltage applied across electrodes of each region depending on the forward bias or the reverse bias of the diode structure of the MQW.
For instance, it is assumed that the first region 51 having a compressive-strained MQW is reverse-biased, the second region 52 having a tensile-strained MQW is forward-biased, and the first region 51 receives incident TE mode light, as shown in FIG. 1A. FIG. 1B shows a dependency of the net gain and TM mode light power generated in the mode converter of FIG. 1A on TE mode light power. With the increase of the incident TE mode light power, as shown in FIG. 1B, the saturable absorption region, i.e., the compressive-strained MQW region 51 more absorbs the TE mode light to decrease the loss of the combined resonator and to increase the net gain. When the net gain exceeds the threshold gain, the amplifying region, i.e., the tensile-strained MQW region 52 lases to generate outgoing TM mode beam. That is, the mode converter of FIG. 1A has a function for converting the incident TE mode light to the outgoing TM mode light.
On the other hand, when the second region 52 having a tensile-strained MQW is reverse-biased whereas the first region 51 having a compressive-strained MQW is forward-biased, the mode converter has a function for converting incident TM mode light to outgoing TE mode light.
In the structure of the conventional TE/TM converter shown in FIG. 1A, it is generally necessary to provide a low reflective coating layer on the incident facet of the combined resonator for obtaining sufficient conversion efficiency. The low reflective coating layer, however, enlarges resonator loss with respect to the lasing TM mode light, and degrades the characteristics of the combined resonator if the outgoing TM mode light returns to the combined resonator at the outgoing facet thereof.